CN109682853B - FBG-based frozen soil ice content distributed in-situ measurement method and device - Google Patents
FBG-based frozen soil ice content distributed in-situ measurement method and device Download PDFInfo
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- CN109682853B CN109682853B CN201910020807.4A CN201910020807A CN109682853B CN 109682853 B CN109682853 B CN 109682853B CN 201910020807 A CN201910020807 A CN 201910020807A CN 109682853 B CN109682853 B CN 109682853B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/20—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
- G01K11/3206—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres at discrete locations in the fibre, e.g. using Bragg scattering
Abstract
The invention discloses a FBG-based frozen soil ice content distributed in-situ measurement method and device, comprising the following steps: a heating power supply, a tubular sensor with a built-in resistance wire and an FBG, an FBG demodulator and a computer for analyzing and processing monitoring data. Burying the manufactured tubular sensor into frozen soil to be measured through direct burying or drilling; connecting a heating power supply to heat a resistance wire arranged in the tubular sensor for a short period, and diffusing heat into surrounding frozen soil through the tubular sensor with good heat conduction performance; the built-in FBG of the tubular sensor senses temperature change, and the wavelength reading of the FBG is acquired and recorded through an FBG demodulator; converting the wavelength data into temperature change to obtain a temperature characteristic value in the heating process; finally, the frozen soil ice content i and the temperature characteristic value delta T established by the calibration test t And (5) obtaining the ice content of the frozen soil by linear relation. The invention can realize the distributed and continuous measurement of the ice content of the frozen soil.
Description
Technical Field
The invention discloses a method and a device for measuring ice content of frozen soil in a distributed in-situ manner by using Fiber Bragg Gratings (FBGs), and relates to the technical field of frozen soil ice content measurement.
Background
Frozen soil is soil with temperature lower than 0 ℃ and ice, and is a multiphase complex system consisting of soil particles, ice, unfrozen water and gas. Frozen soil is very sensitive to temperature and unstable in physical properties, and its characteristics are controlled by the amount of ice in addition to the soil texture, volume weight and water content. Unlike unfrozen soil, ice in frozen soil makes the properties of frozen soil both special and complex. Therefore, the measurement of the ice content of frozen soil has important significance for both theoretical research and engineering practice.
Currently, methods for measuring ice content in frozen soil include an expansion method, a dielectric spectroscopy method, a heat pulse method, and a nuclear magnetic resonance method (NMR).
The basic principle of the expansion method is as follows: the soil sample to be measured is placed in a cylindrical container to be wetted to saturation, and a tubular tensiometer is inserted at one end of the container and extends to the other end of the container. When the soil freezes, the volume of ice formed expands as the soil becomes saturated, and in order to ensure that the soil structure is not destroyed, the expanding volume forces a portion of the liquid water out of the soil pores and through the tensiometer into the pre-calibrated capillary. By measuring the liquid water in the capillary tube and then based on the expansion coefficient of the water frozen into ice, the volume of ice can be calculated.
The basic principle of dielectric spectroscopy is: the ice content is reflected indirectly by measuring the permittivity of the frozen soil, and is usually calculated by determining the dielectric spectra at two frequencies and combining a dielectric mixing model.
The basic principle of the heat pulse method is as follows: when the temperature of the frozen soil is unchanged, the unfrozen water content is no longer changed. The thermal conductivity of the frozen soil is measured by means of the heat pulse, and the ice content of the frozen soil can be calculated. The method is only suitable for the environment with lower temperature, and the measurement result is unreliable when the temperature is close to the freezing point.
The measurement principle of Nuclear Magnetic Resonance (NMR) is: the nuclei of some atoms are like small magnetic rods, and under the action of the external strong magnetic field, the nuclei are oriented and arranged. If applied by radio waves, the atoms will absorb enough energy to rearrange in another stable direction within the applied magnetic field. The prepared frozen soil sample is placed in a pulse analyzer and radio pulses are applied to the emittance of the soil sample. Under the action of the radio pulse, the receiving coil around the soil sample will generate a voltage reflecting the atomic number of the absorbed energy, the magnitude of which is proportional to the atomic number of oxygen in the solid ice in the soil sample. The analyzer can measure the ice content in the frozen soil sample by detecting the voltage.
All the four methods can measure the ice content of frozen soil and have respective limitations: the expansion method is limited in a laboratory environment, so that the original structure of the frozen soil is easily damaged, and the ice content of the frozen soil of the unsaturated soil cannot be measured; the heat pulse method can measure the ice content in an environment with lower temperature, but when the temperature is close to a freezing point, the measured data is not accurate enough; the use of dielectric spectroscopy is limited by the type of soil; the NMR method has higher measurement accuracy, but the analyzer is huge and expensive, and is very complex to install and use, and the application range is limited to laboratory environment only, and cannot be widely used.
Disclosure of Invention
The invention aims to provide a distributed in-situ measurement method and device for frozen soil ice content based on FBG, which are based on a temperature response principle, and the method and device are used for performing temperature sensing on an internally heated pipe body by using an FBG tubular sensor, and determining a temperature characteristic value through a temperature rising curve of the pipe body so as to measure the frozen soil ice content. The method solves the defects that the measurement of the ice content is inaccurate, the original structure of the frozen soil is easy to be damaged, the in-situ measurement of the ice content of the frozen soil is difficult to be carried out, and the like.
In order to solve the problems, the invention adopts the following technical scheme: a FBG-based frozen soil ice content distributed in-situ measurement method comprises the following steps:
step one, implanting a tubular sensor which is completely manufactured and packaged into a corresponding position of frozen soil to be detected, wherein the tubular sensor comprises a tube body, an optical fiber, a heating resistance wire and an FBG sensor;
step two, heating the tubular sensor tube body under the action of current; stopping heating after the diffusion heat flux density is constant, and starting cooling the pipe body;
step three, the FBG demodulator collects and records the heating time interval [ t ] 1 ,t 2 ]Wavelength reading of the internal FBG, t 1 30s, t after stabilizing the diffusion heat flow density 2 The time for beginning to cool the pipe body;
step four, converting the wavelength data into tube temperature information by using a data analysis processing system; calculating a temperature characteristic value of the pipe body, and according to the linear relation between the temperature characteristic value of the pipe body and the ice content of frozen soil: i=k 1 ΔT t +b 1 Calculating ice content of frozen soil, wherein i is ice content of frozen soil, delta T t For measuring the characteristic value, k, of the temperature of the tubular sensor 1 、b 1 Is constant and is determined by calibration tests of a plurality of groups of frozen soil samples; the characteristic value of the temperature of the tubular sensor is that after the diffusion heat flow density around the tubular sensor is constant after the tubular sensor is electrified and heated, a characteristic time interval [ t ] is selected 1 ,t 2 ]The arithmetic mean of the temperatures measured at equal time intervals within a range is different from the initial temperature.
In the first step, a tubular sensor is vertically paved in frozen soil to be tested in a drilling or direct burying mode.
Step two, the heating power of the tubular sensor is constant, so that the steady-state heating of the sensor is ensured; the equal time interval in the fourth step is 10 seconds.
In step three, the time interval [ t ] 1 ,t 2 ]The characteristic time interval is defined, and the value is changed according to different water contents and different soil conditions.
The ice content i of the frozen soil is the ratio of the mass of ice in the frozen soil to the mass of all water; the ice content of the frozen soil sample is determined by nuclear magnetic resonance.
The device used in the frozen soil ice content distributed in-situ measurement method of the FBG comprises a heating power supply, a tubular sensor, an FBG demodulator and an analysis processing monitoring data device, wherein the tubular sensor comprises a tube body, an optical fiber, a heating resistance wire and the FBG sensor, the optical fiber and the heating resistance wire penetrate through the tube body, the optical fiber is provided with a plurality of FBG sensors, the heating resistance wire is connected with the heating power supply through an electrified lead, and the optical fiber is connected with the FBG demodulator through an optical fiber lead and is used for collecting and recording wavelength readings after heating tends to be stable; the analysis processing monitoring data device is connected with the FBG demodulator, and converts wavelength data into tube body temperature information by using the data analysis processing system and calculates a temperature characteristic value of the tube body.
The tubular sensor is made of high-heat-conductivity insulating plastic, and the heat conductivity coefficient is 1.13-1.20W/(m.k).
The tubular sensor adopts a sensitization packaging structure and consists of two semi-cylinders with the diameter of R, a circular small groove with the diameter of R is formed in the middle of the cross section of one semi-cylinder, optical fibers are paved in the small groove, and the two ends of the optical fibers are in a natural relaxation state and are not stressed by tension; injecting non-curing heat-conducting paste into the small groove; and fixing and packaging the two semi-cylinders by epoxy glue, and arranging a fixture to fix the pipe body outside the pipe body at intervals D.
The distance between adjacent FBG sensors in the tubular sensor is d; the tubular sensors are used singly or in series; adjacent tubular sensors are connected with nuts through screw ports at two ends.
And the external optical fiber lead of the tubular sensor is provided with a carbon fiber cloth protective sleeve, and the FBG sensor is connected along the drill hole and the outer wall of the tubular sensor.
The Fiber Bragg Grating (FBG) is a periodic grating manufactured in a fiber core, when the fiber is pulled along the axial direction or the temperature is changed, the fiber is deformed along the axial direction, and the refractive index of the fiber is changed along with the axial direction, so that the output signal spectrum is changed, and the numerical measurement can be realized. The fiber grating sensor is an intrinsic wavelength modulation type sensor developed based on the Bragg condition on the basis of the fiber grating, has the advantages of small volume (the external diameter of a bare FBG sensor is 125 mu m), strong electromagnetic interference resistance, high performance, high stability, corrosion resistance and high sensitivity, and is widely applied to monitoring bridges, dams and geotechnical structures in recent years.
The beneficial effects are that:
1. the invention measures the ice content based on the coefficient of thermal conductivity of frozen soil, and can reduce the influence of moisture migration on a measurement result when measured at a lower temperature.
2. The invention can directly measure the ice content of the undisturbed frozen soil, has small disturbance to the frozen soil, and avoids the structural and component changes of the frozen soil in the sampling, transporting and preserving processes.
3. The invention can realize the distributed and continuous measurement of the ice content of the frozen soil.
4. The invention can realize the real-time monitoring of the ice content change of the frozen soil.
5. The invention has the advantages of economy, safety, convenient operation, strong anti-interference capability, reliable precision and high stability.
6. The method can be applied to experimental researches of different scales, and can improve the spatial resolution in a serial mode.
Drawings
FIG. 1 is a schematic view of the internal longitudinal structure of the distributed in situ measurement sensor according to the present invention.
Wherein, 1, screw; 2. curing the heat-conducting paste; 3. FBG; 4. an optical fiber; 5. an optical fiber protective sleeve; 6. energizing the wire; 7. a resistance wire; 8. a high thermal conductivity insulating tube;
FIG. 2 is a schematic internal cross-sectional view of a distributed in situ measurement sensor according to the present invention.
2, not solidifying the heat conduction paste; 4. an optical fiber; 7. a resistance wire; 8. a high thermal conductivity insulating tube; 9. epoxy glue; r is the diameter of the fiber exit hole inside the sensitization structure; r is the diameter of the circular columnar tube body outside the sensitization structure.
FIG. 3 is a schematic diagram of the overall structure of the distributed in-situ measurement sensor according to the present invention.
Wherein, 10, the nut; 11. FBG tubular sensor; 12. a clamp; 13. an optical fiber outlet; d is the fixture spacing.
Fig. 4 is a schematic diagram of a frozen soil ice content distribution type in-situ measurement system according to the invention.
14, a heating power supply; 15. a computer for analyzing and processing the monitoring data; 16. FBG demodulator; 17. an optical fiber lead; 18. energizing the wire; 19. frozen soil; 20. a tubular sensor; 21. an optical fiber grating; 22. a resistance wire; 23. drilling.
FIG. 5 is a graph showing the calibration result of the linear relation between the characteristic value of the temperature of the tubular sensor in sandy soil and the ice content of frozen soil in example 1.
FIG. 6 is a graph showing the measurement results of the ice content of the sand of example 1.
FIG. 7 is a graph showing the variation of the ice content with the depth of frozen soil in example 2.
Detailed Description
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate the invention and together with the embodiments of the invention, serve to explain the invention. The invention is further explained below with reference to the drawings and the examples.
As shown in fig. 1 and 2, the internal heating tubular sensor for measuring the ice content of frozen soil based on FBG comprises an FBG sensor, an optical fiber, a heating resistance wire, non-solidified heat conducting paste, a tube body with a screw, an optical fiber protective sleeve and an electrified lead from inside to outside.
As shown in fig. 3, an internal heating tubular sensor for measuring frozen soil ice content based on FBG is provided, a fixture fixing tube body is arranged outside the sensor tube body at intervals D, adjacent tube bodies are connected through nuts, and fiber leads are reserved on the nuts to penetrate out of an orifice.
As shown in fig. 4, the FBG-based frozen soil ice content distributed in-situ measurement system comprises a heating power supply 1, a computer 2 for analyzing and processing monitoring data, an FBG demodulator 3 and a tubular sensor 7. The heating power supply 1 keeps constant power, so that the resistance wire works under stable current; the tubular sensor 7 is a tubular sensor of an FBG with an internal heating function; the FBG demodulator 3 is connected with an optical fiber with an FBG sensor and is used for collecting and recording wavelength readings after heating tends to be stable; the computer 2 for analyzing and processing the monitoring data is connected with the FBG demodulator, and converts the wavelength data into tube body temperature information by using the data analyzing and processing system and calculates the temperature characteristic value of the tube body.
The FBG demodulator detects the bragg wavelength reflected by the FBG to reflect the temperature information. In the test, an A-01FBG demodulator manufactured by Suzhou south Intelligence sensing technology Co., ltd is adopted to demodulate the FBG, and wavelength readings are acquired. The demodulator sample recording interval was 10 seconds.
A FBG-based frozen soil ice content distributed in-situ measurement method comprises the following steps:
step one, implanting a tubular sensor which is completely manufactured and packaged into a corresponding position of frozen soil to be detected, wherein the sensor is an FBG tubular sensor with an internal heating function and comprises a tube body, an optical fiber, a built-in resistance wire and an FBG sensor;
step two, connecting a power supply, electrifying and heating the tubular sensor in the step one, and starting heating the tubular body under the action of current; stopping electrifying and heating after the diffusion heat flux density around the sensor is constant, and starting to cool the pipe body;
step three, the FBG demodulator collects and records the heating time t 1 ,t 2 ]Wavelength reading, t, of FBG in interval 1 30s, t after stabilizing the diffusion heat flow density 2 The time for beginning to cool the pipe body;
step four, converting the wavelength data into tube temperature information by using a data analysis processing system; calculating a temperature characteristic value of the pipe body, and according to the linear relation between the temperature characteristic value of the pipe body and the ice content of frozen soil: i=k 1 ΔT t +b 1 Calculating ice content of frozen soil, wherein i is ice content of frozen soil, delta T t For measuring the characteristic value, k, of the temperature of the tubular sensor 1 、b 1 Is constant and is determined by calibration tests of a plurality of groups of frozen soil samples; the characteristic value of the temperature of the tubular sensor is that after the temperature field formed by the tubular sensor after being electrified and heated tends to be stable, a characteristic time interval [ t ] is selected 1 ,t 2 ]The arithmetic mean of the temperatures measured at equal time intervals within a range is different from the initial temperature.
Further, according to the FBG-based frozen soil ice content distributed in-situ measurement method, in the first step, the manufactured and packaged tubular sensor is implanted into the corresponding position of the frozen soil to be measured, and the implantation method comprises the following steps: and vertically paving the sensor in the frozen soil to be measured by adopting a drilling or direct burying mode.
Further, according to the FBG-based frozen soil ice content distributed in-situ measurement method, the heating power of the tubular sensor is stable, and the steady-state heating of the sensor is ensured; the equal time interval in the fourth step is 10 seconds.
Further, the FBG-based frozen soil ice content distributed in-situ measurement method is characterized in that the time interval [ t ] in the third step 1 ,t 2 ]The characteristic time interval is defined, and the value is changed according to different water contents and different soil conditions.
Further, the frozen soil ice content i is the ratio of the mass of ice in the frozen soil to the mass of all water.
Further, the FBG-based frozen soil ice content distributed in-situ measurement method is characterized in that the calibration test comprises the following steps:
step one, a plurality of groups of frozen soil with known ice content;
step two, implanting a tubular sensor which is completely manufactured and packaged into a corresponding position of frozen soil to be detected, wherein the tubular sensor is an FBG tubular sensor with an internal heating function and comprises a tube body, an optical fiber, a heating resistance wire and an FBG sensor;
step three, connecting a power supply, electrifying and heating the tubular sensor in the step two, and starting heating the tubular body under the action of current; stopping electrifying and heating after the diffusion heat flux density around the sensor is constant, and starting to cool the pipe body;
fourth, the FBG demodulator collects and records the heating time t 1 ,t 2 ]Wavelength reading, t, of FBG in interval 1 30s, t after stabilizing the diffusion heat flow density 2 Time for beginning to cool down the tube bodyConverting the wavelength data into tube temperature information;
calculating the temperature characteristic value of each pipe body by using a data analysis processing system, and fitting the linear relation between the temperature characteristic value of each pipe body and the ice content of frozen soil: i=k 1 ΔT t +b 1 Wherein i is ice content of frozen soil, delta T t For measuring the characteristic value, k, of the temperature of the tubular sensor 1 、b 1 Is a constant; the characteristic value of the temperature of the tubular sensor is that after the diffusion heat flow density around the tubular sensor is constant after the tubular sensor is electrified and heated, a characteristic time interval [ t ] is selected 1 ,t 2 ]The arithmetic mean of the temperatures measured at equal time intervals within a range is different from the initial temperature.
Further, according to the FBG-based frozen soil ice content distributed in-situ measurement method, the number of groups of frozen soil with known ice content is 4-6 groups; the ice content of the frozen soil sample is determined by nuclear magnetic resonance.
The frozen soil ice content distributed in-situ measurement device based on the FBG comprises a heating power supply, a tubular sensor, an FBG demodulator and a computer for analyzing and processing monitoring data. The heating power supply keeps constant power, so that the resistance wire works under stable current; the tubular sensor is an FBG tubular sensor with an internal heating function and comprises a tube body, an optical fiber, a heating resistance wire and an FBG sensor; the FBG demodulator is connected with an optical fiber with an FBG sensor and is used for collecting and recording wavelength readings after heating tends to be stable; the computer for analyzing and processing the monitoring data is connected with the FBG demodulator, and the data analyzing and processing system is used for converting the wavelength data into tube body temperature information and calculating the temperature characteristic value of the tube body.
Further, the tube material of the tube sensor is purchased from Kunshan, a company of high-impact insulating materials, and the heat conductivity coefficient of the tube sensor is 1.13-1.20W/(m.k). The high-heat-conductivity insulating plastic has the remarkable characteristics of good heat conductivity, good toughness, light specific gravity, strong insulation, corrosion resistance, ageing resistance and the like.
Further, the frozen soil ice content distributed in-situ measurement device is characterized in that two pore canals are formed in the tube body of the FBG tubular sensor, one pore canal is provided with a heating resistance wire, and the other pore canal is provided with an optical fiber with an FBG sensor.
Further, the frozen soil ice content distributed in-situ measurement device is based on the characteristic that the FBG sensor responds to temperature and strain simultaneously and the property that the optical fiber is easy to break, and the FBG sensor improves the temperature sensitivity coefficient through sensitization encapsulation: the sensitization structure consists of two semicircular columns with the diameter of R, a circular small groove with the diameter of R is formed in the middle of the cross section of one semicircular column, optical fibers are paved in the small groove, and the two ends are in a natural loose state and are not stressed by tension; injecting non-curing heat conducting paste into the small groove to accelerate heat conduction speed, and simultaneously playing a role in buffering or even eliminating strain; and fixing and packaging the two semicircular columns by using epoxy glue, and arranging a clamp to fix the pipe body outside the pipe body at intervals D in consideration of the fact that the length of the pipe body required by distributed in-situ measurement is large.
Further, in the frozen soil ice content distributed in-situ measurement device, 10 FBG sensors are connected in series in the tubular sensor, and the distance between every two adjacent FBG sensors is d; the series connection of a plurality of tubular sensors realizes the quasi-distributed in-situ measurement of the ice content of frozen soil; adjacent pipe bodies are connected with nuts through screw ports at two ends.
Further, the frozen soil ice content distributed in-situ measurement device is characterized in that an external optical fiber lead of the tubular sensor is packaged in a protective way through a carbon fiber cloth protective sleeve, and an FBG sensor is connected along the outer walls of the drill hole and the tubular sensor.
The principle of the invention is as follows: the basic principle of the FBG-based frozen soil ice content distributed in-situ measurement method is to measure the frozen soil ice content by utilizing the linear relation between the temperature characteristic value and the frozen soil ice content in the temperature rising process measured by an FBG sensor. Further it can be explained that: the heat conduction performance of frozen soil is determined by soil particles, gas, unfrozen water and ice, wherein the air heat conduction coefficient is 0.024W/(m.times.K), the water heat conduction coefficient is 0.60W/(m.times.K), and the ice heat conduction coefficient is 2.25W/(m.times.K). The characteristics of the soil particles are kept unchanged in the measuring process, and the thermal conductivity of gas is far smaller than that of unfrozen water and ice, so that the thermal conductivity of gas can be ignored, the total water content of the soil is kept constant, the unfrozen water content is determined by the ice content, and therefore, the thermal conductivity of frozen soil is determined by the ice content. Since the coefficient of thermal conductivity of ice is 3 to 4 times that of water, the higher the ice content, the stronger the heat conductivity of frozen soil. The pipe body FBG sensor with the internal heating function is implanted into frozen soil to be detected, the heating temperature of the pipe body is increased after the pipe body is electrified, a temperature difference is formed between the pipe body and the frozen soil, the heat transfer capacity in the frozen soil with higher ice content is stronger, the total energy generated by the pipe body due to constant power supply is fixed, so that the more the energy diffused into the frozen soil body, the less the energy used for heating the pipe body, and the lower the temperature characteristic value of the pipe body. Therefore, the frozen soil ice content can be obtained by measuring the temperature characteristic value obtained after the pipe body is heated for a certain time.
Example 1
The method and the device are used for carrying out an indoor experiment for measuring the ice content of certain frozen soil.
Calibration test of linear relation between temperature characteristic value of tubular sensor and ice content of frozen soil:
firstly, collecting 5 groups of frozen soil samples with different depths in a frozen soil area through drilling, and measuring the ice content of the frozen soil with each depth by using a nuclear magnetic resonance method, wherein the measurement results are 3%, 4.6%, 8.7%, 9.3% and 11.5%;
step two, using a large-volume ring cutter (the height of the ring cutter is more than or equal to 4cm, and the volume is more than or equal to 120 cm) 3 ) Sampling, and implanting a tubular sensor with complete manufacture and encapsulation into the center position of frozen soil to be detected, wherein the tubular sensor is an FBG tubular sensor with an internal heating function and comprises a tube body, an optical fiber, a heating resistance wire and an FBG sensor;
step three, connecting a power supply, electrifying the tubular sensor in the step two, and starting heating the tubular body under the action of current; stopping electrifying and heating after the diffusion heat flow density around the pipe body is constant, and starting cooling the pipe body;
step four, the FBG demodulator collects and records wavelength readings every 10 seconds, and converts the wavelength data into tube temperature information;
fifthly, drawing a heating curve of the tubular sensor by using a data analysis processing system, and selecting temperature characteristicsThe interval is after heating [14min,25min ]]Calculating the temperature characteristic value of each pipe body in the characteristic interval, and combining the frozen soil ice content fitting obtained in the previous step to obtain the linear relation between the temperature characteristic value of each pipe body and the frozen soil ice content: i= -0.709 Δt t +0.1406, the calibration coefficient R obtained by linear fitting 2 =0.998, as shown in fig. 5.
According to the same method and steps of the calibration test, the power-on heating, the information acquisition and demodulation, the temperature information analysis and the temperature characteristic value calculation are carried out, and the obtained temperature characteristic value is substituted into a linear function i= -0.709 delta T determined by the calibration test t In +0.1406, the ice content of frozen soil is calculated as shown in FIG. 6. As can be seen from FIG. 6, the frozen soil has an ice content of 8.2% as measured using the method of the present invention.
In the test process of this embodiment 1, the frozen soil sample and the FBG tubular sensor are both placed in a freezing chamber having the same temperature as the sampling ground, and the heating power supply, the FBG demodulator and the computer for analyzing and processing the monitoring data are all placed outside the freezing chamber.
Example 2
The method and the device are used for carrying out in-situ experiments for measuring the ice content of the frozen soil in a distributed manner, and monitoring the ice content of the frozen soil at different depths. And selecting frozen soil in a sandy soil area of Qinghai at a test site.
Step one, performing calibration test on the linear relation between the temperature characteristic value and the ice content of sandy soil in the region according to the method and the step described in the embodiment 1, and fitting to obtain the linear relation between the temperature characteristic value and the ice content;
step two, 13 FBG tubular sensors are vertically buried in the frozen soil to be detected in series through drilling, wherein each FBG tubular sensor is formed by connecting 10 FBG sensors in series, the distance between the adjacent sensors is 20cm, and the ice content of the soil with different depths can be measured simultaneously;
connecting the tubular sensor to a stable-power supply and an FBG sensor; connecting a power supply, electrifying and heating the FBG tubular sensor in the second step, and starting heating the tube under the action of current; stopping electrifying and heating after the diffusion heat flux density around the sensor is constant, and starting to cool the pipe body;
step four, the FBG demodulator automatically collects and records wavelength readings every 10 seconds, and converts the wavelength data into tube temperature information;
and fifthly, calculating temperature characteristic values sensed by all FBG sensors by using a data analysis processing system, substituting the obtained temperature characteristic values into a primary function determined by a calibration test, and calculating the ice content of frozen soil at each depth to obtain a change curve of the ice content of the frozen soil along with the depth, as shown in fig. 7.
The theoretical derivation process of the linear relation between the temperature characteristic value of the tubular sensor and the ice content of the frozen soil in the embodiment is described as follows:
assuming that the frozen soil to be measured has uniformity, the pipe body is positioned in an infinite soil layer with consistent initial temperature. Simplifying the heat transfer in such frozen soil into a one-dimensional problem. The unit area is taken on the surface of the tube body, and according to ohm's law, the energy generated in unit time of the unit area is as follows:
Q 1 ＝I 2 R (1)
q in formula (1) 1 The energy generated per unit area of the resistance wire is represented by I, which is a current, and R, which is the resistance of the resistance wire. I. R is a known constant, Q 1 And therefore is also constant.
According to energy conservation, the energy used for heating the tube body per unit time is expressed as:
Q 2 ＝C m (T-T 0 )＝C m ΔT t (2)
q in (2) 2 Is the energy for heating the pipe body; c (C) m Is the specific heat capacity of the pipe body; t (T) 0 The temperature of the heating front pipe body; t is the measured temperature of the heated pipe body, and the value is the average temperature of the pipe body after the diffusion heat flow density around the pipe body is constant; delta T t Defined as a temperature characteristic value.
Heat quantity Q dissipated per unit area of pipe body in unit time:
Q＝Q 1 -Q 2 ＝I 2 R-C m ΔT t (3)
the heat source is regarded as a wireless long-line heat source, and the temperature field of frozen soil around the tube body meets the following conditions:
in the formula (5), r is a radius taking the pipe body as the center of a circle; t is heat exchange time; t (r, T) is the temperature of the rock-soil body at the position with the radius r from the center of the pipe body at the moment T; q is the heat exchange amount of the pipe body; l is the length of the pipe body; lambda is the heat conductivity coefficient of the rock-soil body; a is the thermal diffusivity of the rock-soil body; beta is a calculation process parameter; t (T) ∞ The temperature of the rock-soil body (namely the initial temperature of the stratum) at infinite distance from the center of the pipe body; c (C) S Is the heat capacity of the unit volume of the rock-soil body.
Assuming steady-state heat conduction of the pipe body, r takes the value of the radius r of the pipe body w Then the temperature T measured after heating:
based on the wireless long-line heat source model, the logarithm of T and time T is linear according to the formula (6) by using a linear derivation method. Can be simplified as:
T＝klnt+m (7)
wherein T is the average temperature after the heating and stabilization in the tube body; p is the thermal resistance of the pipe body; gamma is Euler constant, and 0.577216 is taken; k and m are the relationship between the temperature and time measured by the FBG interrogator, and the slope and intercept of the straight line are obtained based on least square fitting.
The heat dissipation Q in combination with the temperature response can give the thermal conductivity:
the frozen soil consists of a soil framework, gas, unfrozen water and ice, so that the thermal conductivity of the frozen soil consists of four parts
λ＝λ s s+λ g g+λ l l+λ i i (11)
Wherein s and g are the ratio of soil particles, gas mass and total mass of the frozen soil, and l and i are the ratio of unfrozen water, ice mass and total water mass in the frozen soil. The gas content g is extremely small and can be ignored, and the heat conductivity coefficient lambda of the soil skeleton, unfrozen water and ice is high s 、λ l 、λ s And the content s of soil particles can be obtained by related experiments and data, the unfrozen water content l=1-i, so that the frozen soil thermal conductivity is a linear function of the ice content, and can be expressed as:
λ＝ai+b (12)
in formula (12), a=λ i ,b＝λ s s+λ g g+λ l l, a and b are constants.
The combined type (3) (10) and (12) can obtain the characteristic value delta T of the ice content and the temperature t Linear relation of (c):
can be further simplified to obtain:
i＝k 1 ΔT t +b 1 (14)
wherein the method comprises the steps ofk 1 、b 1 Are constant.
From equation (14), it can be seen that the characteristic value of the temperature of the tubular sensor is in a linear relationship with the ice content of the frozen soil. The frozen soil ice content can thus be calculated by monitoring the temperature of the tubular sensor.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention and are intended to be included within the scope of the present invention.
Claims (8)
1. FBG-based frozen soil ice content distributed in-situ measurement method is characterized by comprising the following steps of
Step one, implanting a tubular sensor which is completely manufactured and packaged into a corresponding position of frozen soil to be detected, wherein the tubular sensor comprises a tube body, an optical fiber, a heating resistance wire and an FBG sensor;
step two, heating the tubular sensor tube body under the action of current; stopping heating after the diffusion heat flux density is constant, and starting cooling the pipe body;
step three, the FBG demodulator collects and records the heating characteristic time interval [ thet 1 ,t 2 ]Wavelength reading of an internal FBG, saidt 1 For 30s after the diffusion heat flux density was stabilized,t 2 the time for beginning to cool the pipe body;
step four, converting the wavelength data into tube temperature information by using a data analysis processing system; calculating a temperature characteristic value of the pipe body, and according to the linear relation between the temperature characteristic value of the pipe body and the ice content of frozen soil:calculating ice content of frozen soil, wherein +_>For frozen soil ice content, < >>For the characteristic value of the temperature measured by the tubular sensor, < >>、/>Is constant and is determined by calibration tests of a plurality of groups of frozen soil samples; the characteristic value of the temperature of the tubular sensor is that after the diffusion heat flow density around the tubular sensor is constant after the tubular sensor is electrified and heated, a characteristic time interval [ t ] is selected 1 ,t 2 ]The arithmetic mean of the temperatures measured at equal time intervals within a range is different from the initial temperature.
2. The FBG-based frozen soil ice content distributed in-situ measurement method according to claim 1, wherein in the first step, a tubular sensor is vertically laid in frozen soil to be measured in a drilling or direct-burying mode.
3. The FBG-based frozen soil ice content distributed in-situ measurement method according to claim 1, wherein the tubular sensor in step two has constant heating power, and the steady-state heating of the sensor is ensured; the equal time interval in the fourth step is 10 seconds.
4. The FBG based frozen soil ice content distributed in-situ measurement method according to claim 1, wherein the time interval [ t ] in step three 1 ,t 2 ]The characteristic time interval is defined, and the value is changed according to different water contents and different soil conditions.
5. The FBG based frozen soil ice content distributed in situ measurement method according to claim 1, wherein the frozen soil ice contentIs the ratio of the mass of ice in the frozen soil to the mass of all water; the ice content of the frozen soil sample is determined by nuclear magnetic resonance.
6. The device used in the frozen soil ice content distributed in-situ measurement method of the FBG according to any one of claims 1-5 is characterized by comprising a heating power supply, a tubular sensor, an FBG demodulator and an analysis processing monitoring data device, wherein the tubular sensor comprises a tube body, an optical fiber, a heating resistance wire and the FBG sensor, the tube body structure adopts sensitization packaging and consists of two semi-cylinders with the diameter of R, a circular small groove with the diameter of R is formed in the middle position of the cross section of one semi-cylinder, the optical fiber is paved in the small groove, the two ends of the small groove are in a natural relaxation state, and non-curing heat conducting paste is injected into the small groove; the tube body material adopts high heat conduction insulating plastic, the tube body internally penetrates through an optical fiber and a heating resistance wire, a plurality of FBG sensors are arranged on the optical fiber, the heating resistance wire is connected with a heating power supply through an electrified lead wire, and the optical fiber is connected with an FBG demodulator through an optical fiber lead wire and is used for collecting and recording wavelength readings after heating tends to be stable; the analysis processing monitoring data device is connected with the FBG demodulator, and converts wavelength data into tube body temperature information by using the data analysis processing system and calculates a temperature characteristic value of the tube body.
7. The apparatus for use in a method of distributed in situ measurement of ice content of frozen soil of FBGs according to claim 6, wherein adjacent FBG sensors within the tubular sensor are spaced apart by a distance d; the tubular sensors are used singly or in series; adjacent tubular sensors are connected with nuts through screw ports at two ends.
8. The device for the frozen soil ice content distributed in-situ measurement method of the FBG according to claim 6, wherein the carbon fiber cloth protective sleeve is arranged outside the external optical fiber lead of the tubular sensor, and the FBG sensor is connected along the drill hole and the outer wall of the tubular sensor.
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| CN111879794A (en) * | 2020-07-28 | 2020-11-03 | 中国科学院西北生态环境资源研究院 | Monitoring system for ice content of regional surface soil |
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